1. Field of the Invention
The subject invention generally pertains to snap springs and more specifically to those used in conjunction with electrical contacts.
2. Description of Related Art
Heretofore it has been well known to provide overcenter snap springs for electrical control devices such as thermostats and switches. Burch U.S. Pat. Nos. 3,213,228; 4,032,734; 4,424,506; and 4,796,355 disclose how to stress a flat, M-shaped spring member (M-blade) to become snap acting by spreading the inner legs of its U-shaped loops with an activating member, thus side-stressing the planar spring member and causing it to become snap acting and bistable.
The applicant of the present invention has designed, used, reduced to practice and commercialized numerous products using the M-blade for over 15 years and has become expert at Burch "M-blade" technology in the process. As a result of this work, the inventor of the present application has discovered and observed numerous limits to the operating of the M-blade, the remedy of which are the objects of this application for patent.
One such observation is the need to spread apart the inner leg portions of the M-blade spring member to insert the activating member (rivet, pin or screw) prior to stressing, thus slowing the assembly operation. For example, if a commercial rivet was to be used to accomplish the stressing, the rivet could not be inserted without first forcing the inner leg portions apart to allow passage for the rivet through the opening. In addition, the rivet and opening must be made to very close tolerances, as a small change in size will cause a large change in stress and consequently a large change in force required to snap the blade from one position into the other. Furthermore, the riveting process must be very closely monitored. Any excessive pressure and deformation in the area where the M-blade spring member is riveted to its spring arm will cause unwanted mechanical bias and non-uniform snap-action from assembly to assembly in production. Still further, the M-blade spring member may turn out of alignment if torque is applied to it, caused for example by shock or vibration. Further yet, the Burch invention combines stressing and attaching the M-blade spring member to a spring arm in one operation, thus preventing effective and separate control of each operation during the manufacturing process.
Subsequent Burch U.S. Pat. No. 4,796,355 attempts to overcome these limitations and disadvantages in providing an alternate method for stressing the M-blade spring member by compressing the outside legs of the U-shaped loops and locking them with a folded strip of metal welded to the spring member at each end of the folded strip. In practical use, as for example in an electrical switch, the folded strip of metal may need to be made from a suitable contact material such as a silver alloy. To conduct electrical current the M-blade snap acting spring member is made from copper alloy, due to its low resistivity. Experience with attempting to use the Burch invention has shown that welding such a folded precious metal strip to the conductive copper alloy presents difficulty in welding. To overcome these difficulties the folded precious metal strip may need to be made from a composite material which on the one side facilities welding and on the other side provides good electrical contact properties. The method for producing this snap-contact assembly, as envisioned by its inventors has proven to be difficult to achieve and prohibitive in cost.
A second observation is the flexing or bending of the M-blade spring member, particularly in the longitudinal plane, when force is applied to make it snap overcenter. As a result of this flexing, optimal snap action of the M-blade spring member is limited by its material thickness.
A third observation is the absence of anti-rotation means designed to prevent the M-blade spring member attached to the spring arm, to move out of position as a result of material expansion and contraction under combined and adverse environments such as temperature, vibration and shock.
A fourth observation is that the method of using a rivet, screw or other mechanical compression assembly means, for mounting and attaching the M-blade spring member to the spring arm can lead to an increase of the electrical resistance path between the M-blade spring member and the spring arm assembly when exposed for a long time to adverse environments and high temperatures as they are encountered in many industrial sites.
A fifth observation is the effect on the M-blade operating characteristics and its snap acting ability of the ratio existing between loop centers of the M-blade and distance from the loop centers to the anchor point at which the M-blade is attached to its supporting arm.